Due to their high degree of covalent bonding, silicon carbide materials exhibit many superior physical and chemical properties, especially in high temperature structural applications, than many other ceramic materials such as aluminum oxide (Al.sub.2 O.sub.3), zirconium (ZrO.sub.2), and silicon nitride (Si.sub.3 N.sub.4), etc. These advantageous properties include high hardness and thermal conductivity and excellent thermal shock resistance, hot strength, acid resistance, abrasion resistance, and a high creep limit. Because of these advantageous properties, their applications have been expanded to areas that include the fabrication of mechanical shaft seals, valves, pump linings, shaft bearings, semi-conductor heat treatment tubes, heat exchanger, and various molds for use in steel and other industries. However, silicon carbide materials also suffer from the disadvantages of having relatively low fracture toughness and flexural strength, resulting in inadequate reliability of mechanical components made therefrom. Therefore, it has become highly desirable to improve the toughness as well as the strength of the silicon carbide materials.
Generally, there are two approaches to improve the toughness of silicon carbide. The first approach involves compositing silicon carbide with a fibrous whisker-like crystalline or granularly shaped reinforcing second phase so as to form a reinforced composite. This compositing approach, though has been shown to improve the strength and toughness of silicon carbide, it also introduced many unexpected problems, such as the difficulty to a form dense sintered composite body, the relatively high production cost, and the concern of the cancer-related carcinogenicity associated with some of the reinforcing materials, especially with the extraneous whisker-like crystalline additives.
The second approach involves self-reinforcement. In this approach, beta phase silicon carbide powder is used as a starting material. During the high temperature sintering process, a phase transformation occurs and the beta (cubic) phase silicon carbide will be transformed into a more thermally stable non-cubic hexahedral alpha phase silicon carbide. By carefully controlling the crystal growth process, the hexahedral crystal will be subject to an unequiaxial growth, thus forming elongated plate-like grains resembling the whisker-like structure. However, because the phase change and crystal growth take place in a very rapid pace, the sintering condition must be extremely carefully controlled, and only a very narrow window of sintering temperature and sintering time is allowed. A precipitous decrease in the material strength will be observed if the sintering process is not properly controlled such that plate-like crystals are allowed to grow too long. The self-reinforcement approach is further complicated by the low diffusion coefficient of the silicon carbide material (because of their very high covalent bonding) which requires a very high sintering temperature (greater than 2,000.degree. C.) to obtain dense sintered product.
Because of the low diffusion coefficient of silicon and carbon atoms in the silicon carbide material, it is relatively difficult to achieve dense sintered product using pressureless sintering technique. A number of sintering aids have been disclosed in the prior art so as to obtain high density (greater than 90% of the theoretical limit) sintered silicon carbide products. U.S. Pat. No. 4,124,667 discloses a pressureless sintering process of silicon carbide to produce silicon carbide ceramic bodies by firing shaped bodies, which contain a mixture of silicon carbide, boron carbide, phenolic resin, and a temporary binder, at a sintering temperature of from about 1,900.degree. C. to 2,500.degree. C. The density of the silicon carbide ceramic bodies so produced achieves 75% of the theoretical densities.
U.S. Pat. No. 4,135,938 discloses a dense silicon carbide ceramic body with improved thermal shock resistance which comprises a pressureless sintered composition comprising silicon carbide, and from about 0.3 to about 3.0 weight percent of aluminum diboride.
U.S. Pat. No. 4,179,299 discloses pressureless sintered silicon carbide ceramic bodies which consist essentially of (a) about 91 to 99.85 weight percent, preferably at least 99 weight percent, of silicon carbide, wherein at least 95 percent by weight of the silicon carbide is of the alpha phase; (b) up to about 5.0 weight percent of carbonized organic material; (c) from about 0.15 to about 3.0 percent by weight of boron; and (d) up to about 1.0 percent by weight of additional carbon. The silicon carbide ceramic bodies have at least 90 percent equiaxed microstructure.
U.S. Pat. No. 4,237,085 discloses a method of pressureless sintering a silicon carbide mixture to obtain a sintered, dense product wherein the silicon carbide starting material does not contain a densification aid, such as boron, beryllium or aluminum. The silicon carbide mixture consists essentially of particulate silicon carbide containing less than about 6.0 percent by weight of carbon in the form of elemental carbon or a carbon source material. The sintered silicon carbide products can achieve a density greater than about 85% of the theoretical density.
U.S. Pat. No. 4,564,490 discloses a series of sintered silicon carbide bodies comprising 0.027 to 11.300 atomic percent of sintering assists, which contain one of more members of rare earth (Be, Mg, Ca, Se, or Ba) oxides, and the balance of silicon carbide.
U.S. Pat. No. 4,354,991 discloses a process for producing a dense sintered silicon carbide ceramic body which comprises the steps of first molding a mixture of an oxygen-containing aluminum compound which can be converted into aluminum oxide by heating in a non-oxidative atmosphere at a ratio of 0.5 to 35 weight percent as Al.sub.2 O.sub.3, with the remaining ceramic material substantially being silicon carbide; then pressureless sintering the mixture in a non-oxidative atmosphere at 1,900.degree. to 2,300.degree. C. The sintered silicon carbide bodies have a flexural strength of at least 25 kg/mm.sup.2.
U.S. Pat. No. 4,230,497 discloses dense sintered shaped articles of polycrystalline alpha-silicon carbide consisting of (a) at least 95.4 weight percent of alpha-silicon carbide; (b) about 0.1 to 2.0 weight percent of additional carbon; (c) about 0.2 to 2.0 weight percent of aluminum; (d) about 0 to 0.5 weight percent of nitrogen; and (e) about 0 to 0.1 weight percent of oxygen; the alpha-silicon being in the form of a homogeneous microstructure with an average grain size of less than 10 microns. The sintered polycrystalline alpha-silicon carbide bodies have at least 97 percent of their theoretical density.
U.S. Pat. No. 4,855,263 discloses a process for preparing silicon carbide sintered body by adding and mixing powders of silicon carbide having an average grain size of no more than 5 microns with 0.1 to 5 weight percent of magnesium boride and 0.1 to 5 weight percent of carbon or an organic compound producing the same quantity of carbon; shaping the resulting mixture into a predetermined form; and firing the resulting shaped body at a temperature of 1,900.degree. to 2,300.degree. C., under vacuum or in an inert gas atmosphere.
U.S. Pat. No. 4,692,418 discloses a process of making sintered silicon carbide/carbon composite ceramic bodies by firing a microporous shaped green body, which has been infiltrated with an organic material, a sintering aid selected from the group consisting of aluminum, beryllium and boron compounds, silicon carbide having a surface area of from 5 to 100 m.sup.2 /g, and, optionally a temporary binder, at a sintering temperature of about 1,900.degree. to about 2,300.degree. C. The sintered silicon carbide/carbon composite body has a homogeneous very fine grain microstructure with at least 50 percent of its silicon carbide grains having a size not exceeding about 5 microns and an aspect ratio less than about 3, with graphite grains having an average size not exceeding that of the silicon carbide.
U.S. Pat. No. 4,526,734 discloses a process for producing a sintered silicon carbide body which comprises the steps of (a) preparing a sintering raw material consisting essentially of a substantially non-soluble silicon carbide fine powder and a sintering aid into a dispersion medium selected from the group consisting of benzene, cyclohexane and water, together with a substance selected from the group consisting of a molding assistant and a deflocculating agent; (b) forming a uniform suspension of the sintering raw material; (c) spray freezing the suspension in an atmosphere held at a temperature lower than a melting temperature of the dispersion medium to obtain frozen granulates; (d) free drying the frozen granulates to obtain a powdery dried mixture; (e) shaping the powdery dried mixture into a green body; and (f) shaping the green body with pressing.
In Taiwan Pat. Pub. No. 79109721, it was disclosed a process for preparing silicon carbide bodies having high toughness and fracture resistance comprising the steps of preparing a homogeneous mixture containing about 82 to 99.4 weight percent silicon carbide (primarily of alpha-phase), about 0.5 to 10 weight percent of aluminum nitride, about 0.1 to 8 weight percent of a rare earth oxide, and a temporary binder; and pressureless sintering the mixture at a temperature of about 1,775.degree. to 2,200.degree. C. in an inert environment. The sintered silicon carbide body exhibited a fracture toughness of at least 7 MPa-m.sup.1/2, measured based on a precrack width of 0.5 mm.
In all the references discussed above, the contents of which are expressly incorporated by reference, although dense sintered silicon carbide bodies can be obtained with its density achieving 95% of the theoretical limit, the sintering temperature must be increased to between 2,000.degree. and 2,500.degree. C. At this high temperature, which causes the silicon carbide grains to experience a rapid anisotropic growth, very long plate-like crystals will likely to be formed. This results in an observable decrease in the strength of the silicon carbide material.
Recently, a number of publications have discussed the use of alumina and yttria, instead of the traditional additives of boron, carbon, aluminum, beryllium, rare earth metals, etc., as sintering aid. In the article entitle "Toughening Behavior of Silicon Cargide with Additions of Yttria and Alumina," J. Am. Ceram. Soc., 73 [5] 1431-34 (1990), Kim et al studied the fracture-toughness mechanism of silicon carbide with additions of yttria (Y.sub.2 O.sub.3) and alumina (Al.sub.2 O.sub.3). They reported that significant crack deflection had occurred, and median deflection angles increased with increased volume fractions of the second phase, which was accompanied by increased fracture toughness. The silicon carbide used in their study was alpha-phase silicon carbide.
In U.S. Pat. No. 4,569,921 it is disclosed a process for making sintered silicon carbide moldings by using as a sintering aid a composition comprising oxides of at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sin, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Pm and LU, or at least one element selected from the group consisting of Li, Be, Mg, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Zr, Nb, Mo, Ba, Tc, Tc, W and Th, in addition to the used known sintering aids such as rare earth element oxides, boron oxide and aluminum oxide.
In "Preparation of Pressureless-Sintered SiC-Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3," J. Materials Science 23 3744-3749 (1988), Omori et al reported that sintering additives prepared from aluminum hydroxide and yttrium hydroxide were soluble in water and resulted in a binder. When a beta-silicon carbide powder was mixed with the sintering additive and sintered at 2,150.degree. C. without pressure, the oxides formed from the additive promoted sintering, and the sintered body contained no pores. Aluminum, silicon, and yttrium oxide were precipitated in the sintered body. The bulk density and flexural strength of the sintered silicon carbide bodies were 3.11 g/cm.sup.3 and 470 MPa, respectively.
More recently, in an article entitled "Microstructural Development and Mechanical Properties of Pressureless-Sintered SiC with Plate-Like Grains Using Al.sub.2 O.sub.3 -Y.sub.2 O.sub.3 Additives," Lee et al reported that dense SiC ceramics with plate-like grains were obtained by pressureless sintering using beta-SiC powder with the addition of 6 weight percent Al.sub.2 O.sub.3 and 4 weight percent Y.sub.2 O.sub.3. It was reported that during the sintering of the beta-SiC powder compact, the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the beta-alpha transformation. With increased holding time, the extent of the beta-to-alpha phase transformation, the grain size, and the aspect of grains, increased, and the fractured toughness increased from 4.5 MPa-m.sup.1/2 to 8.3 MPa-m.sup.1/2. It was concluded that the crack deflection and crack bridging were considered to be the main operative mechanisms that led to the improved fracture toughness.
In the sintering processes discussed above, the contents of which are expressly incorporated by reference, the sintering temperature can be reduced to between 1,800.degree. and 2,000.degree. C. while achieving the goal of obtaining a dense sintered product. The lowered sintering temperature also helped controlling the crystal growth to as to limit the length of the plate-like crystals to less than about 15 microns, thus improving the fracture toughness of the sintered product. However and unfortunately, the predominance of the presence of the metallic plate-like crystalline structure also adversely affects the flexural strength of the sintered products. Thus, it remains to be developed a process which can simultaneously improve both the fracture toughness and the flexural strength of the silicon carbide material, while maintaining a high density of the sintered product and confining the sintering process under a pressureless condition.